This invention relates to free-space optical telecommunications, and more particularly to automatic gain control for free-space optical telecommunications links.
Free-space optical telecommunications offers an attractive alternative to hard-wired or radio communication in certain situations. For example, a telecommunications services provider who wants to enter a new geographical area may have little or no hard-wired plant in that area and may wish to avoid the cost and complexity of installing such plant to serve the new area. Similarly, radio communications resources are limited and regulated, and a new telecommunications services provider may not have sufficient rights to use those resources in a new geographical area.
Free-space optical telecommunication is therefore attractive because it avoids the need for hard-wired plant and because, unlike radio telecommunication, it is essentially unregulated. Optical telecommunication also has the advantage of very large information capacity. Thus optical telecommunications links can support a wide range of telecommunications services such as telephone, video, audio, and computer data transmission.
A possible problem with free-space optical telecommunication is that it is subject to time-varying attenuation through the atmosphere. For example, infrared or other light may scintillate at frequencies up to about 200 Hz as it passes through the atmosphere. Digital modulation of the light is one way to render free-space optical information transmission more immune from these atmospheric effects. However, digital modulation tends to increase transmission cost for at least some types of information, especially information which is initially in analog form and which is ultimately used in analog form. This is true, for example, for most telephone information and much video (television) information.
In view of the foregoing, it is an object of this invention to improve free-space optical telecommunication.
It is a more particular object of this invention to reduce the deleterious effects on analog, free-space, optical telecommunication of atmospheric disturbances such as scintillation.